Highly sensitive balances in particular, such as microbalances and ultra-microbalances, analytical balances or precision balances, are subject to influence factors which can lead to measurement deviations over the course of time. Such balances therefore have to be checked on a regular basis in order to ensure that they produce accurate weighing results. Such checks, so-called routine tests which are performed within a regulatory framework, are officially required in particular for balances used in the fields of pharmacology, biotechnology and food technology and are set down in FDA regulations (Food and Drug Administration, U.S. Department of Health and Human Services). However, the manufacturers of balances also recommend to their customers that balances used in commercial applications be checked at regular intervals.
To determine deviations one uses check weights with defined nominal values. According to norm standards, for example the internationally recognized recommendation R111 published by OIML (Organisation Internationale de Métrologie Légale), these kinds of check weights are subject to tolerance limits within which the actual weight values have to lie in relation to the nominal weight value. Under this tolerance system, the weights are divided into different weight classes according to different precision requirements. For example, the tolerance limit for a one-milligram weight in class E1 (the highest accuracy class) is ±0.003 mg, while the tolerance limit in class M1 (the lowest accuracy class applicable to a one-milligram weight) is ±0.2 mg.
Check weights, as the term is used in the present context, should be understood to include weights of all kinds that are used to check and/or calibrate and/or certify balances or weights particularly in areas that are subject to regulatory control. These check weights are occasionally also called verification weights or calibration weights.
Check weights can be made of one solid piece or of several pieces of material. Single-piece check weights are made of one block of material, while check weights composed of more pieces have a cavity on the inside which is filled with so-called adjustment material up to the point where the nominal weight has been attained, whereupon the cavity is closed off. It should be noted, however, that check weights made up of a plurality of pieces are not permitted in the highest accuracy classes according to OIML.
As the actual weight values of check weights will change over time due to wear, this could have the consequence—in cases where these check weights are used to check balances—that weighing results or industrial processes may also run outside their tolerance limits. One must therefore make certain that in any given case the check weight tolerance is being met. To accomplish this purpose, the check weights themselves are regularly checked against other check weights, so-called verification standards. The time intervals for such verification checks are dependent on the respective accuracy class of the weights or on the area of application and the particular circumstances of the application.
For each individual check weight, a certificate is issued on request, which states the actual weight value at the specific time, the nominal weight value, the accuracy class relative to a given class limit, as well as a calibration I.D. number and the number of the calibration certificate. Each time another verification check, a so-called recalibration, is performed at a later date, a new certificate is issued in which a new certificate number is assigned to the same weight, but the same calibration I.D. number remains assigned to the weight.
The check weights or sets of check weights with different weight values are stored in special weight container cases for the distribution and later, at their place of application, for storage by the user. In such a container case, there are appropriately dimensioned seating recesses provided for each weight denomination, so that for example a 100-gram weight can be set with a precise fit only into the recess for 100-gram weights, but not into a recess for a 50-gram weight, while it would not completely fill out the recess for a 200-gram weight, so that a correlation between weights and recesses is possible based on size. The certificates of the individual weight pieces are placed into these container cases so that in principle the connection between certificate and check weight is established. This is normally made evident by means of a label that is affixed to the container case, on which the calibration I.D. number is printed, and a further label on which the certificate number is printed.
Due to the manual handling of the check weights in the process of performing the aforementioned routine tests, it is however easily possible that the connection between the weight piece and its associated certificate and/or its calibration I.D. number gets lost. This can happen for example if a balance is to be certified or calibrated for 400 grams and if for this purpose—because there is no 400-gram weight available—a 200-gram weight piece and two 100-gram weight pieces are used instead. Regardless of whether the two 100-gram weight pieces are stored in the same container case or come from two different container cases, it is possible that handling errors will occur in the process, resulting in a mix-up of the two 100-gram pieces. The consequence of this is a wrong match between certificate and weight piece, which cannot even be effectively checked, so that an error of this kind remains undiscovered.
This method has the problem that there is no definite correlation that ties the certificate to the check weight, i.e. to the physical weight piece itself. The handling of such check weights therefore requires the utmost diligence in order to ensure that the correct match between certificate and calibrated weight piece is permanently preserved. Still, there is no guarantee of achieving this goal. Inadvertent mix-ups cannot be ruled out, nor can they be reliably detected after the fact.
In German laid-open application DE 40 06 375 A1, the concept of equipping check weights with a code marking that represents the weight value is disclosed. This is realized by electronically storing the weight value in an electronic circuit which is contained in the weight piece itself. This has the disadvantage that electrical contacts are necessary for the transmission of the data from the weight piece to the balance and vice versa and that because of these contacts, the weight has to be set in a defined position and, in particular, special devices are required which make the manufacture and use an error-prone process. Also, an electronic data storage is not totally error-resistant, so that data errors due to inappropriate handling of the check weights or also due to material fatigue, and thus calibration errors which occur as a result, cannot be completely ruled out in this case either. Furthermore, check weights of this kind are expensive to produce.
Since the identification marking only contains the initial actual value, this coding system does not provide an individual identification of each weight piece, but only a classification according to weight value. Under the method described in this reference, an individual weight piece can be traced back only insofar as the highest possible number of weight checks that can be performed is entered in the electronic data storage device of the weight and each weight check is counted until this upper limit is reached. Traceability beyond this time frame or in regard to other attributes such as place and date of manufacture, production lot number, etc., is impossible. A recall campaign which could be necessary for example in case of a manufacturing error in a production lot is therefore not possible for check weights that are identified in this way.
It is therefore an objective to advance the design of a check weight in such a way that the weight is permanently and individually traceable.